1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus and a semiconductor integrated circuit that turn on a discharge lamp such as a cold cathode fluorescent lamp used for, for example, a liquid-crystal display device.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a discharge lamp lighting apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-196196 and FIG. 2 is a view showing waveforms during a burst dimming operation of the apparatus according to the related art shown in FIG. 1. The apparatus of FIG. 1 conducts the burst dimming operation by changing ON/OFF intervals of a driving switching transistor 325 in a DC-DC converter 32 to alternate ON/OFF states of a discharge lamp (fluorescent lamp) 34.
According to the related art, a dimming pulse signal P1 is low during an ON period (lit-up state) of the burst dimming operation that conducts an intermittent oscillation operation to dim the discharge lamp 34. During the ON period in which the dimming pulse signal P1 is low, transistors 42 and 43 are OFF, and therefore, an inverting terminal of a comparator 323 receives a dead-time voltage DT1. An error signal ER becomes higher than the voltage DT1, and accordingly, the comparator 323 supplies a control signal CS to the transistor 325. This results in increasing an ON ratio of the transistor 325. Namely, a power supply period (duty) for the discharge lamp 34 gradually increases to perform a soft start action. The soft start action gradually increases voltage and current applied to the discharge lamp 34, to prevent an excessive stress on the discharge lamp 34.
During an OFF period (lit-out state) of the burst dimming operation, a current that is insufficient to turn on the discharge lamp 34 is passed through a transformer 331 arranged in an inverter (automatic) 33. This prevents a sharp change in current supplied to the transformer 331 when the discharge lamp 34 is turned on from the OFF state. As a result, a turn-on action during the burst dimming operation can quickly activate the discharge lamp 34 from the soft start action.